The invention relates to an operating mechanism of a low voltage multipole circuit breaker with high electrodynamic strength and comprising an electrical power circuit having, per pole, a pair of compensated contact means held in the closed position by electrodynamic compensation effect of the repulsion forces.
A mechanism of the kind mentioned is described in the document EP-A-222,645 filed by the applicant and comprises a toggle device associated to a tripping hook and to an opening spring to drive the movable contact to an open position when the hook is actuated from a loaded position to a tripped position, a switching bar made of insulating material coupled to the toggle device extending transversely to the frame and comprising a rotary shaft supporting the movable contact means of all the poles, an opening ratchet operating in conjunction with the tripping hook to perform loading or tripping of the mechanism, respectively in the locked position or in the unlocked position of said ratchet, and a latching lock operated by a tripping part to actuate the opening ratchet to the unlocked position. The electrodynamic strength of the circuit breaker results from the action of the contact pressure springs on the multiple fingers, and of the compensated contact means, whose articulation axis is subjected to strong mechanical reactions. The mechanism is able to absorb these reactions for a maximum short-circuit current threshold. Beyond this threshold, the reactions are liable to damage certain axes or transmission means of the mechanism and to increase the tripping force at the level of the stage comprising the hook, the opening ratchet and the latching lock. Operation of the instantaneous circuit breaker requires a response time of about 10 ms to obtain tripping of the mechanism, which is too long, if the performances of the circuit breaker have to meet the requirements of high electrodynamic strength and a breaking capacity greater than 130 kA.
It has already been proposed to use the mechanical reaction arising from electro-dynamic compensation of the compensated contact means to bring about automatic tripping (see document EP-A-0,780,380). The opening ratchet comprises disengageable actuating means bringing about self-unlocking of the lock in the presence of a short-circuit current exceeding a calibration threshold defined by flexible means, said self-unlocking being commanded from a mechanical reaction generated by the electrodynamic compensation effect and causing ultrafast rotation of the lock to unlock the opening ratchet before the tripping part operates.
The circuit breaker obtained has very good performance as far as electrodynamic strength is concerned as self-unlocking is in practice calibrated for high current levels, notably greater than 180 kA peak. To obtain a sufficient breaking capacity, it is however necessary for the pole and its extinguishing chamber to have very large dimensions to the detriment of the general size and of the price.